Processors have traditionally been difficult to design and to modify. For this reason, most systems that contain processors use ones that were designed and verified once for general-purpose use, and then used by multiple applications over time. As such, their suitability for a particular application is not always ideal. It would often be appropriate to modify the processor to execute a particular application's code better (e.g., to run faster, consume less power, or cost less). However, the difficulty, and therefore the time, cost, and risk of modifying an existing processor design is high, and this is not typically done.
Another difficulty with prior art processor design stems from the fact that it is not appropriate to simply design traditional processors with more features to cover all applications, because any given application only requires a particular set of features, and a processor with features not required by the application is overly costly, consumes more power and is more difficult to fabricate. In addition it is not possible to know all of the application targets when a processor is initially designed. If the processor modification process could be automated and made reliable, then the ability of a system designer to create application solutions would be significantly enhanced.
Configurable processors having easily modified or extended instruction sets and architectures to enhance or customize functionality of the processor can be used to accommodate specific application requirements. Configurability allows the designer to specify whether or how much additional functionality is required for a product. The simplest sort of configurability is a binary choice: either a feature is present or absent. For example, a processor might be offered either with or without floating-point hardware.
Though, once the processor has been designed, its operation must be verified. Processors generally execute instructions from a stored program using a pipeline with each stage suited to one phase of the instruction execution. Therefore, changing or adding an instruction or changing the configuration may require widespread changes in the processor's logic so each of the multiple pipeline stages can perform the appropriate action on each such instruction. Configuration of a processor requires that it be re-verified, and that this verification adapt to the changes and additions. This is not a simple task. Processors are complex logic devices with extensive internal data and control state, and the combinations of control and data and program make processor verification a demanding art. Adding to the difficulty of processor verification is the difficulty in developing appropriate verification tools. Since verification is not automated in prior art techniques, its flexibility, speed and reliability is less than optimal.
In addition, once the processor is designed and verified it is not particularly useful if it cannot be programmed easily. Processors are generally programmed with the aid of extensive software tools, including compilers, assemblers, linkers, debuggers, simulators and profilers. However, when the processor changes, the software tools used to program the processors must change as well. It does no good to add an instruction if that instruction cannot be compiled, assembled, simulated or debugged. The cost of software changes associated with processor modifications and enhancements has been a major impediment to flexible processor design in the prior art.
Thus, it is seen that prior art processor design is of a level of difficulty that processors generally are not typically designed or modified for a specific application. Also, it can be seen that considerable improvements in system efficiency are possible if processors could be configured or extended for specific applications. Further, the efficiency and effectiveness of the design process could be enhanced if it were able to use feedback on implementation characteristics, such as power consumption, speed, etc., in refining a processor design. Moreover, in the prior art once a processor is modified, a great deal of effort is required to verify the correct operation of the processor after modification. Finally, although prior art techniques provide for limited processor configurability, they fail to provide for the generation of software development tools tailored for use with the configured processor.